The invention relates to an osteoinductive material and to a process for preparing said material.
Calcium phosphates such as hydroxyapatite are known to be osteoconductive, or bioactive. This means that they act as a template along which bone growth can occur. Further, bone formation can directly take place at the surface of the material, and a strong bond is obtained with bone tissue. Osteoinductivity, on the other hand, is regarded as a property of materials that induce the formation of bone tissue. In the past, this property has only been described in connection with materials that contain osteoinductive, proteinaceous factors such as bone morphogenetic proteins (BMP""s).
Recently, however, several studies have been reported that indicate a possible osteoinductive capacity of calcium phosphates when implanted intramuscularly in dogs or baboons. Generally, it is assumed that the presence of a porous structure and a specific geometry of the implant plays a crucial role in the osteoinductive character of the implant.
Yamasaki et al., in Biomaterials 1992, vol. 13, no. 5, 308-312, have described to have found heterotopic bone formation around porous hydroxyapatite ceramic granules, but not around dense granules. The porous granules had a size between 200 and 600 xcexcm, and a continuous and interconnected microporosity ranging in diameter from 2 to 10 xcexcm.
The present invention aims to provide a material having an improved osteoinductivity. It is an object of the invention to provide a material that is suitable to be used as an implant in living organisms and to function as a (temporary) substitute for bone tissue. Thus, the material should be both biocompatible and biodegradable.
Surprisingly, it has been found that this object is achieved by the provision of a ceramic material having both macropores and micropores of specific sizes. Hence, the invention relates to an osteoinductive biomaterial, which is based on a ceramic material and which has a total porosity of 20 to 90%, wherein macropores are present having a size ranging from 0.1 to 1.5 mm, and wherein micropores are present having a size ranging from 0.05 to 20 xcexcm.
The material of the invention shows excellent osteoinductive behaviour in living tissue. The formation of bone tissue at the surface of the material of the invention assists in a favourable acceptation of an implant made of said material. Moreover, the formation of the bone tissue accelerates the recovery of any damage in the bone structure, which forms the reason for applying the implant.
An osteoinductive biomaterial according to the invention is based on a ceramic material. The biomaterial may for instance be a medical implant formed of a ceramic material. It is also possible that the biomaterial is a medical implant of a different material, such as a metal or a polymeric material, on which the ceramic material is present in the form of a coating. Another possibility is described by M. L. Gaillard and C. A. van Blitterswijk in J. Mater. Sci., Materials in Medicine, 5:695-701 (1994). This possibility concerns a copolymer having hydrogel-like properties, which may be calcified in the presence of calcium and phosphate ions.
In principle, any ceramic material that is both sufficiently biocompatible and sufficiently biodegradable to be used as an implant in living tissue can be used. Preferably, the ceramic material is capable of providing a calcium phosphate surface, either in vitro or in vivo, which has the present specific surface structure. It is further preferred that the ceramic material is capable of adsorbing biologically active agents, such as growth factors (BMP""s etc.), either in vitro or in vivo. Suitable examples of ceramic materials include calcium phosphates, glass ceramics and materials containing calcium phosphates and/or glass ceramics.
Preferably, the ceramic material is a calcium phosphate. Preferred calcium phosphates are octacalcium phosphate, apatites, such as hydroxyapatite and carbonate apatite, whitlockites, such as a-tricalcium phosphate and xcex2-tricalcium phosphate, and combinations thereof.
An important aspect of the invention is the physical structure of the osteoinductive biomaterial. The material comprises both macropores and micropores. The total porosity ranges from 20 to 90%, preferably from 40 to 70%.
The macropores of the material have a size of from 0.1 to 1.5 mm. Preferably, the size of the macropores lies between 0.2 and 1 mm. It has been found that the indicated sizes of the macropores have a significant beneficial influence on the osteoinductive character of the material. Further preferred is that the macropores are interconnected.
The micropores of the material have a size of from 0.05 to 20 xcexcm. Preferably, the micropores are at least located in the macropores. In accordance with this embodiment, the formation of bone tissue is highly promoted. A preferred range for the size of the micropores is from 0.5 and 10 xcexcm. In a preferred embodiment, the micropores are at least present in the surface of the macropores. The microporosity of the material""s surface preferably lies between 40 and 60%.
In accordance with the invention, the biomaterial preferably consists of crystals. Preferably, the size of the crystals is similar to the size of the micropores. When this is the case, the biomaterial has a preferable microrugosity. Thus, the size of the crystals lies preferably between 0.05 and 20 xcexcm, more preferably between 0.5 and 10 xcexcm.
The osteoinductive biomaterial according to the invention may advantageously be used in applications where bone formation is desired. Thus, the material may be used for the manufacture of medical implants, particular implants for bone substitution. The material may further be used for the manufacture of a scaffold for tissue engineering a bone equivalent.
The invention further relates to processes for preparing an osteoinductive biomaterial as described above.
In a first embodiment, the osteoinductive biomaterial may be prepared by sintering a ceramic material under such conditions, that an osteoinductive biomatieral as described above is obtained. The ceramic material is, before the sintering, in a calcined state. The sintering is preferably performed at a temperature between 1000 and 1275xc2x0 C., treated with an aqueous solution of an organic acid and washed to remove the acid.
Preferably, the sintering is carried out at a temperature between 1150 and 1250xc2x0 C. The duration of the sintering step may suitably be chosen between 6 and 10 hours, preferably between 7 and 9 hours. It has further been found advantageous to perform the sintering while the ceramic material is submersed in a powder of the ceramic material. This beneficially affects the reactivity of the surface of the material, and consequently also the bioactivity (dissolution, re-precipitation).
After the sintering, the material is preferably ground with sandpaper, such as Sixe2x80x94C sandpaper, to remove chemical surface impurities.
Subsequently, the material is treated with an aqueous solution of an acid. Suitable acids in this regard are any etching acids, i.e. any acids which lead to a slight dissolution of the calcium phosphate based material. The use of the following acids has been found to lead to extremely favourable results: maleic acid, hydrochloric acid, phosphoric acid, and combinations thereof. The concentration of the acid in the solution is preferably chosen such that the pH of the solution lies between 0 and 4, more preferably between 1 and 3.
After the acid treatment, which preferably lasts between 3 and 15 minutes, the ceramic material is washed to remove the acid. The washing may suitably be performed using ethanol, water or a combination thereof.
Finally, it is preferred to subject the obtained osteoinductive biomaterial to a sterilisation treatment, such as a steam sterilisation.
In a second embodiment, a slurry of a powder of the ceramic material in an aqueous solution of a negative replica forming agent, which during sintering burns or evaporates, is sintered under such conditions that an osteoinductive biomaterial as described above is obtained. Suitable negative replica forming agents include hydrogen peroxide, baking powder or bicarbonate. Preferably, hydrogen peroxide is used.
Thus, first, the powder is added to an aqueous solution of the negative replica forming agent to form a slurry. The concentration of the negative replica forming agent in the slurry preferably lies between 0.5 and 15 wt. %, more preferably between 1 and 5 wt. %, based on the weight of the solution. The powder is added in a ratio of between 0.5 to 5, preferably 1 to 3 grams per 1 milliliter of the solution. The slurry may then be cast in a mould having a desired shape and size and sintered. The sintering is preferably carried out at a temperature between 800 and 1300xc2x0 C., more preferably between 1000 and 1200xc2x0 C. for a period of up to 12 hours. Care should be taken that the sintering period is not so long that a dense material is obtained.